The nucleotide biosynthetic pathways provide a rich source of drug targets such as dihydrofolate reductase, the target of the widely used antibacterial drug trimethoprim. IMP dehydrogenase (IMPDH) presents a similar therapeutic opportunity. Although IMPDH inhibitors are used in immunosuppressive, cancer and antiviral therapy, as yet IMPDH inhibitors have not been exploited in antibacterial applications because no bacterial-selective IMPDH inhibitors have been identified. We have been engaged in a medicinal chemistry program to develop IMPDH-targeted drugs for treating the category B parasite Cryptosporidium parvum. We have 'in hand'low nanomolar inhibitors of C. parvum IMPDH (CpIMPDH) with >250 selectivity versus the human enzymes. Surprisingly, CpIMPDH is most closely related to bacterial IMPDHs, suggesting that C. parvum obtained this gene via horizontal transfer. We have identified a structural motif that defines susceptible enzymes;this motif is found in a wide variety of pathogenic bacteria, including seven other select agents. We propose a program to develop the CpIMPDH inhibitors as broader spectrum antibiotics. To this end, we will determine the efficacy of the CpIMPDH inhibitors against a panel of pathogenic bacteria chosen to define the spectrum of action in terms of the structural variation of the target enzyme and the permeability of the bacteria (select agents in bold): Bacillus anthracis (Gram positive), Francisella tularensis (Gram-negative facultative intracellular), Listeria monocytogenes (Gram- positive facultative intracellular), Burkholderia mallei/pseudomallei (Gram-negative facultative intracellular), Staphylococcus aureus (Gram-positive) and Acinetobacter baumannii (Gram-negative). These pathogens pose some of the most serious threats to human health;B. anthracis, Bu. mallei/pseudomallei, and F. tularensis are top priorities for countermeasure development and methicillin-resistant S. aureus (MRSA) and A. baumannii present major treatment challenges.